Oftentimes when a contractor is asked to design a configuration such as an addition to a building or a swimming pool having a curved profile, the focal point of the arc segment defining the curved portion is available and accessible to mark out the curved portion. Under such circumstances, the contractor can access the focal point to plot out the curved portion of the new structure and design accordingly. Unfortunately, there are many instances where an obstacle obstructs access to the focal point. In these cases the obstacle presents difficulties in accurately laying out the desired arc segment. Presently, there are no satisfactory devices or procedures to facilitate the geometric construction of an arc segment when the focal point is unavailable, such as where a building or other obstacle is in the way of reaching the focal point.